Abstract: Electrospray ionization multi-stage tandem mass spectrometry(ESI-MS~n) is applied to analysis of five cephalosporins,including cefuroxime,cefalotin sodium,cefathiamidine,cefoselis and cefoxitin sodium,which are one classes of antibiotics widely used in the prevention and treatment of infectious diseases in the recent years.And their fragmentation pathways are described.The cleavage of C-X(X=O,N and S) in the site of C-3 is the characteristic fragmentation pathway of cephalosporins,both in the negative and positive ion mode.Additionally,in the MS~2 spectra of negative ion mode of cefuroxime,β-lactam cleaves between C6-7 and C8-N1 which is the same as reference papers described.In the MS~n spectra of positive ion mode of cefuroxime,cefalotin sodium,cefoselis and cefoxitin sodium,a neutral loss of CO happens at the side chain of C-7.